Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere.
Evaporative leaks may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system may be isolated at an engine-off event. The pressure in such a fuel system will increase if the tank is heated further (e.g. from hot exhaust or a hot parking surface) as liquid fuel vaporizes. As a fuel tank cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation is monitored and leaks identified based on expected vacuum development or expected rates of vacuum development. The EONV leak test may be monitored for a period of time based on an available battery charge.
However, the EONV leak test is prone to false failures based on ambient conditions, as the rates of vacuum development are based in part on the ambient temperature. During mild weather conditions, the ambient temperature may restrict the amount of heating or cooling of the fuel tank following engine shut-off, and thus limit the rate of pressure or vacuum development. Unexpected changes in temperature can also influence fuel system temperature. Other weather conditions, such as precipitation, barometric pressure, high wind conditions, etc. may influence fuel tank temperature, and thus skew the pressure/vacuum buildup in a sealed fuel system. An EONV test result is thus unreliable if the ambient weather conditions are unknown. Further, changes in ambient weather that are unfavorable to EONV testing may result in aborted or indeterminate tests, reducing the test execution rate. Initiating an EONV test based on the ambient conditions at that time may not be adequate to predict the conditions during the testing duration. Further, the vehicle control module may be operating in a low-power mode during the test, so as to conserve battery charge. It may thus not be feasible to record ambient conditions during the test.
The inventors herein have recognized the above issues and have developed systems and methods to at least partially address them. In one example, a method for a vehicle, comprising: initiating an engine-off natural vacuum test based on an ambient temperature change potential over a testing duration. The ambient temperature change potential may be based on historic weather data and further based on forecast weather data. In this way, the EONV test may be executed only when conditions favor an in-tank temperature change significant enough to cause a threshold change in fuel tank pressure. This, in turn may reduce the number of aborted tests, increasing the overall performance metrics for the test.
In another example, a system for a vehicle, comprising: a fuel system coupled to an evaporative emissions system; and a controller configured with instructions in non-transitory memory, that when executed cause the controller to: responsive to an indication to perform an engine-off natural vacuum test, and further responsive to a vehicle-off event, retrieving historic weather data and forecast weather data from an off-board computing system; determining an ambient temperature change potential over a testing duration; and initiating the engine-off natural vacuum test based on the ambient temperature change potential. In this way, an accurate profile of ambient conditions may be obtained, leading to a more accurate prediction of fuel tank pressure changes over the testing duration. In turn, this may lead to a relaxation of other EONV test entry conditions while maintaining or improving the overall execution rate of the test.
In yet another example, a method for validating an evaporative emissions system leak test, comprising: performing an engine-off natural vacuum test for a testing duration; storing a result of the engine-off natural vacuum test at a controller; responsive to a vehicle-on event, retrieving historic weather data for the testing duration from an off-board computing system; and validating the result of the engine-off natural vacuum test based on the retrieved historic weather data. By retrieving weather data for the testing duration following the EONV test, an accurate estimation of expected fuel tank pressure changes may be obtained. Further, mitigating weather conditions may be detected, and tests occurring during such conditions may be discarded. In this way, the number of false test results may be decreased.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.